The ability to sense and measure a magnetic field is important in many areas. For example, magnetic sensors may be used for compassing, navigation, magnetic anomaly detection, and identifying position. As a result, magnetic sensors may be found in medical, laboratory, and electronic instruments; weather buoys; virtual reality systems; and a variety of other systems.
Such applications frequently employ magnetoresistive (“MR”) sensors capable of sensing small magnetic fields. MR sensors are often formed using integrated circuit fabrication techniques and are typically composed of a nickel-iron (permalloy) thin film deposited on a silicon wafer, or other type of substrate, and patterned as resistive strips. The resistance of the strips varies with respect to an angle formed between a sensed magnetic field and current direction within the sensor. The strip resistance is maximized when the magnetic field and the current direction are parallel to each other.
During the manufacture of an MR sensor, the easy axis (the preferred direction of magnetization) is set to a direction along the length of the film to allow the maximum change in resistance of the strip. However, the influence of a strong magnetic could rotate the magnetization of the film, changing the sensor's characteristics. Following such changes, a strong restoring magnetic field can be applied to the sensor to restore, or set, the sensor's characteristics.
In certain designs, large external magnets can be placed adjacent to the sensor to set the sensor's characteristics. However, such an implementation may not be feasible when the MR sensor has already been packaged into a system. Particularly, some applications require several sensors within a single package to be magnetized in different directions. In such applications, instead of using large external magnets, individual coils may be wrapped around each sensor to restore the sensor's characteristics. Alternatively, current straps, also known as set-reset straps, may be used to restore the sensor's characteristics. The use of current straps in a magnetic field sensing device is discussed in U.S. Pat. No. 5,247,278 to Bharat B. Pant, assigned to the same assignee as the current application. U.S. Pat. No. 5,247,278 is fully incorporated herein by reference.
Another type of magnetic sensor is a giant magnetoresistive (“GMR”) sensor. GMR sensors are typically employed in applications that require measurements of relatively small magnetic fields. GMR sensors may be manufactured using thin film technology and may include multiple layers of alternating ferromagnetic and non-magnetic materials. Generally, a GMR sensor includes two magnetic layers separated by a non-magnetic layer. The resistance of the magnetic layers is related to the direction of magnetization between the two magnetic layers.
Some of the structures currently being used to fabricate GMR elements include unpinned sandwich, antiferromagnetic multilayer, spin valve structures, and spin dependent tunnel structures.
The unpinned sandwich structure may include two magnetic layers separated by a conducting non-magnetic layer. For example, an unpinned sandwich structure may consist of two permalloy layers separated by a layer of copper.
An antiferromagnetic multilayer structure may consist of multiple repetitions of alternating conducting magnetic layers and non-magnetic layers. In this structure, each magnetic layer may have a direction of magnetization antiparallel to the direction of magnetization of the magnetic layers on either side.
Spin valve structures may include a pinned magnetic layer and a free magnetic layer, with a nonmagnetic layer, such as copper, located between the two magnetic layers. The pinned layer may have a fixed magnetization direction, while the free layer may rotate in the presence of an external magnetic field.
Spin dependent tunnel structures are similar to spin valve structures; however, the non-magnetic layer is a non-conductive material, such as an oxide, and current flows from one magnetic layer to another magnetic layer through a tunnel current in the non-conductive layer.
Magnetic field sensors using GMR elements are often fabricated in a Wheatstone bridge configuration. A Wheatstone bridge can be fabricated using four GMR elements, such as spin valves. One of the biggest challenges of fabricating a spin valve GMR sensor in a Wheatstone bridge configuration is producing two GMR element pairs that respond differently to the same external magnetic field. For a spin valve GMR sensor, the direction of magnetization of the pinned layers in adjacent legs of the bridge should be antiparallel in order to utilize the GMR ratio fully.
U.S. Pat. No. 5,617,071 entitled “Magnetoresistive structure comprising ferromagnetic thin films and intermediate alloy layer having magnetic concentrator and shielding permeable masses” discloses one approach to fix the direction of magnetization of the pinned layers in adjacent legs to be antiparallel by shielding one pair of Wheatstone bridge elements. By shielding opposing GMR elements with a highly permeable material, the shielded pair may not experience the effects of an applied magnetic field that rotates the direction of magnetization of the non-shielded pair. However, this approach limits the range of the output signal, reduces sensitivity of the sensor in half, and requires extra processing steps to fabricate the shielded layer.
U.S. Pat. No. 5,561,368 (hereinafter referred to as the '368 patent) entitled “Bridge Circuit Magnetic Field Sensor Having Spin Valve Magnetoresistive Elements Formed on Common Substrate” discloses another approach for producing two GMR element pairs that respond differently to the same external magnetic field. According to the '368 patent, four GMR spin valve elements are formed on the same substrate. The free layers of all four of the spin valve elements have their magnetization axes parallel to one another. The pinned layers of two spin valve elements have their magnetization axes antiparallel to the direction of magnetization of the pinned layers of the other two spin valve elements.
The magnetic field sensor in the '368 patent further includes an electrically conductive fixing layer (a current strap) formed on the substrate. The application of current through the fixing conductor during fabrication of the sensor fixes the direction of magnetization of two of the pinned layers to be antiparallel to the direction of magnetization of the other two pinned layers. While the current is applied to the fixing conductor, the sensor is first heated and then cooled.
The application of the current during the sensor fabrication may be difficult and not feasible, especially when many sensors are fabricated on a single wafer. Multiple power supplies may be required to supply the current to the fixing conductors or individual sensors may have to be linked. Thus, the methods described in the '368 patent may require a complicated manufacturing process. Furthermore, applying heat during the process may reduce the GMR ratio for the material.
Therefore, a need exists for a simple method of setting the magnetization directions in the pinned layers of spin valve GMR sensors configured in a Wheatstone bridge configuration.